The one thing you DON'T want to do is put the money in an account under your SSN. Doing that will constitute a gift from your mother to you (presumably in excess of $12K) and she will have to file a gift tax return. Instead, you can open an account under her name where she can give you the privilege of managing the investments.

Under current tax law, she can give $12K/year gift tax-free to as many people as she wants. Additionally, she can pay the qualified education expenses of your son without having to report that as a gift.